The present application is the national stage under 35 U.S.C. 371 of PCT/JP00/02349, filed Apr. 11, 2000.
The present invention relates to cyanine dyes, and more particularly, to non-symmetric trimethine cyanine dyes which exert excellent recording characteristics when used in high-density optical recording media.
As coming into this multi-media age, optical recording media such as compact disc recordable (CD-R, a write-once memory using compact disc), and digital versatile disc (DVD-R, a write-once memory using digital video disc), have been highlighted. optical recording media can be classified roughly into those of inorganic ablation type which have recording layers composed of inorganic substances such as tellurium, selenium, rhodium, carbon, or carbon sulfide; and those of organic ablation type which have recording layers composed of light absorbents containing organic dye compounds as main ingredients.
Among these, optical recording media of organic ablation type are usually prepared by dissolving a cyanine dye in an organic solvent such as 2,2,3,3-tetrafluoro-1-propanol (abbreviated as xe2x80x9cTFPxe2x80x9d hereinafter), coating the solution on the surface of a polycarbonate substrate, drying the solution to form a recording layer, forming (i) a reflection layer made of a metal such as gold, silver or copper (ii) and a protection layer made of an ultraviolet ray hardening resin on the surface of the recording layer, and successively attaching the above layers (i) and (ii) on the surface of the recording layer. When compared with inorganic ablation type optical recording media, those of organic ablation type have the drawback that their recording layers may be easily changed by environmental lights such as reading- and natural-lights. Organic ablation type optical recording media, however, have the merit that they can be manufactured at a lesser cost because their recording layers can be formed by preparing solutions of light absorbents and directly coating the solutions on the surface of substrates. Organic ablation type optical recording media have now become predominantly low-cost optical recording media because they are mainly composed of organic substances so that they are substantially free of corrosion even when contacted with moisture or sea water and because information, which is stored in optical recording media, can be read by using compact disc players by the establishment of thermal deformation type optical recording media which are a kind of organic ablation type optical recording medium.
What is urgently required in organic ablation type optical recording media is to more increase their recording capacity to suit to this multi-media age. The research for such an increment, which is now being eagerly continued in this field, is to shorten the wavelength of 775-795 nm, that is irradiated by conventional GaAlAs semiconductor lasers, to a wavelength of 660 nm or shorter in order to increase the recording capacity per one side to a level of 4.7 giga bytes (GB) or more. However, since most of conventional cyanine dyes explored for CD-Rs could not appropriately write and read information using a laser beam with a wavelength of 660 nm or shorter when used in high-density optical recording media such as DVD-Rs, the cyanine dyes now used could not fulfil the need for high-storage densification required in many fields.
As another causative for spoiling the high-storage densification of organic ablation type optical recording media, there exist problems of the thermal decomposition and the heat resistance of dyes. In organic ablation type optical recording media, pits are formed by using heat generated when dyes absorb laser beam and then melt and decompose. However, the difference between the melting point and the decomposition point of most of conventional cyanine dyes is quite large and their thermal difference is quite high; the pit formation by laser beam is not sharp or uniform, and the heat of melting and decomposition will conduct to area around the irradiated points and then distort the previously-formed adjacent pits. In addition, most of conventional cyanine dyes have a rather lower decomposition point, and this results in the problem that the part around the pits and other pit-less part on the recording surface may be easily deformed by the accumulated heat which is generated when the dyes are exposed to a reading laser-beam for a relatively-long period of time because the cyanine dyes have a relatively-low heat resistance.
In view of the foregoing, the object of the present invention is to provide an organic dye compound which exerts excellent recording characteristics when used in high-density optical recording media, and to provide uses thereof.
To attain the above object, the present inventors eagerly studied and screened compounds. As a result, they found that non-symmetric cyanine dyes (may be called xe2x80x9ccyanine dyesxe2x80x9d hereinafter) obtainable through a step of reacting either 3,3-dimethyl-5-nitroindolium compounds or 3,3-dimethyl-5-sulfonamideindolium compounds bearing a reactive methyl group or an appropriate leaving group, with 3,3-dimethylbezoindolium compounds bearing a reactive methyl group or an appropriate leaving group, have absorption maxima in the visible region, and substantially absorb a visible light with a wavelength around 650 nm when in a thin layer form. They also found that, unlike conventional related compounds, most of the cyanine dyes of the present invention have the following features: They have only decomposition points or decomposition points undistinguishable from their melting points, have significantly-higher decomposition points and heat resistance than those of conventional related compounds, and promptly decompose at temperatures around their decomposition points. The present inventors confirmed that the cyanine dyes form minute pits stably on the recording surfaces promptly and at a relatively-high density when irradiated with a laser beam at a wavelength around 650 nm in optical recording media. The present invention was made based on the creation of novel organic dye compounds and the discovery of their industrially-useful characteristics.